The mammalian macrophage is an actively motile cell which play a central role in the body's response to invading microorganisms and to cell damage. An understanding of the mechanisms by which this cell is able to move, how this movement is directed up a chemotactic gradient, how the movement is stopped once the site of inflammation is reached and how secretion and phagocytosis are switched on concomitantly are of importance. Actomyosin has been implicated to have a role in a wide range of basic cellular processes which include cell motility, phagocytosis, pinocytosis, exocytic secretion, intracellular transport and cytokinesis. To function in such a wide range of processes it is evident that subtle control mechanisms must exist within the cell to regulate specially and temporally, the interaction between actin and myosin, their assembly into filaments, and disassembly to monomers. The macrophage represents a excellent system in which to study these processes since it is a rich source of both actin and myosin and uses them extensively. Published data and my preliminary results show that these proteins may be readily purified and may be phosphorylated in vitro with purified kinases. This will allow the effects of the phosphorylation of myosin, on binding to actin, on actin-activated Mg2+-ATPase and on filament formation to be investigated. By studying the phosphorylation state of myosin in actively motile or phagocytosizing cells, by immunocytochemical localization of the various phosphorylated forms of myosin and by introduction of specific antibodies which functionally affect the myosin into the cell, by microinjection or electroporation, it will be possible to begin to ascertain the role of these mechanisms in the intact cell. This will lead to a greater understanding of the functioning of the macrophage in combating infection and as a component of the immune response, and will open the possibility of stimulating and directing the bodies own defenses against infection.